This invention relates to drills and more particularly to drill controls for automatically changing the speed of a bit in response to the depth of the bit in a workpiece.
Oftentimes a workpiece to be drilled is made up of a number of layers of different materials. The different layers may consist of difficult to drill materials sandwiched with easier to drill materials. For example, a typical workpiece (such as in current aerostructures) might consist of one greater inch of titanium that requires a drill speed of 360 rpm followed by an inch of aluminum, which should be drilled at 4000 rpm, and then a quarter inch of stainless steel, which also must be drilled at a lower speed. If the aluminum is drilled at the low speed required for the titanium, it takes a substantial amount of time to drill through the aluminum. If, however, the aluminum can be drilled at 4000 rpm, it takes less than one tenth of the amount of time that it otherwise would.
Therefore, a need exists for a drill which automatically changes the speed of the bit in response to the depth of the bit in the workpiece.